Nonaqueous electrolyte cell

ABSTRACT

An anode with a multi-layer structure including a first layer ( 21 ) having carbon as a main component, and a second layer ( 22 ) having lithium-ion conductivity and including a material as a main component thereof which can insert and extract lithium ions, or a multi-layer structure including a third layer ( 23 ) containing lithium in addition to the first layer and the second layer, and a lithium secondary battery including the same. The lithium secondary battery can be provided in which the battery capacity is substantially enhanced in the voltage range where the battery is actually used while maintaining the higher charge-discharge efficiency and the excellent cycle performance.

TECHNICAL FIELD

The present invention relates to a secondary battery, and morespecifically to a lithium secondary battery and a method of fabricatingthe secondary battery.

BACKGROUND ART

The widespread use of mobile terminals such as a cellular phone and anotebook-size personal computer recognizes the importance of secondarybatteries acting as their power sources. These secondary batteries arerequired to be compact, light and high in their capacities, and to haveperformance hardly deteriorated after the repetition of the charging anddischarging.

Although metal lithium may be used as the anode of the secondary batteryin view of its higher energy density and lightness, a problem arisesthat needle crystals (dendrite) are deposited on the lithium surface ofthe metal lithium anode during the charging in the progress of thecharge-discharge cycle, and the crystals penetrate the separator tocause the internal short-circuit, thereby shortening the battery life.

Further, use of lithium alloy having a composition formula of Li_(x)M(“M’ is metal such as Al) as an anode is investigated. A problem alsoarises that pulverization of the anode is caused with the progress ofthe charge-discharge cycle thereby shortening the battery life becausethe lithium metal anode is swollen and contracted with the insertion andextraction of the lithium ion though it has a larger volume for thelithium ion insertion and extraction per a unit volume and thus has alarger capacity.

When a carbon material such as graphite and hard carbon capable ofinserting and extracting the lithium ion is used as the anode, theenergy density becomes lower because the graphite material has a lowercapacity than the metal lithium and the lithium alloy, and the hardcarbon has a larger irreversible capacity on the initial charging anddischarging to decrease the charge-discharge efficiency though thecharge-discharge cycle is excellent.

A number of investigations have been conducted for the purpose ofimproving the anode.

JP-A-2000-21392 discloses the electric contact between an anodecontaining a carbon material and metal such as Si, Ge and Sn or itsoxide, and metal lithium during the fabrication of the battery topropose the improvement of the anti-over discharge performance and thecycle performance.

JP-A-11(1999)-135120 discloses use of a carbon material coated withparticles made of Al, Sn or Sb as an anode to propose the improvement ofthe higher capacity, the higher voltage and the cycle performance.

JP-A-10(1998)-21964 discloses use of a material mainly containingchalcogen compounds having Al, Sn or Si or its oxide as an anode topropose the higher capacity, the elevation of the cycle performance andthe improvement of the production efficiency.

JP-A-2000-182602 discloses a secondary battery anode including an anodesheet made of an amorphous oxide capable of inserting and extractinglithium and laminated with a metal foil mainly made of lithium topropose the higher capacity and the improvement of the anti-overdischarge performance.

JP-A-2001-15172 discloses a secondary battery anode including an anodesheet made of a carbon material laminated with a metal foil mainly madeof lithium to propose the higher capacity and the improvement of thecharge-discharge efficiency.

However, these prior arts cause the following problems.

The techniques described in JP-A-2000-21392, JP-A- 11(1999)-135120 andJP-A-10(1998)-21964 can hardly increase the energy densities of thebatteries sufficiently high because the metals and the metal oxides havethe higher irreversible capacities on the initial charging anddischarging and the larger anode weights. When the metal is mixed withthe carbon-based material, the operating voltage becomes lower comparedwith an anode made of only carbon, and the higher operating voltage canbe hardly obtained because a voltage plateau which is typical to themetal appears on a discharge curve at a voltage higher than that ofcarbon so that the higher operating voltage can be hardly obtained. Thelower limit voltages are fixed depending on uses in the lithiumsecondary battery. The decrease of the operating voltage narrows theusable region so that the capacity increase in the region where thebattery is actually used can be hardly intended.

In the method described in JP-A-2000-21392, the added lithium is reactedwith active functional groups on the carbon surface, adsorbed water onthe carbon surface or moisture contained in the solvent or theelectrolyte to form a film on the anode surface. The lithium containedin the film is electrochemically inactive and cannot contribute to thecharge-discharge capacity so that the improvement of thecharge-discharge efficiency is insufficient. The electric resistances ofthe films are large to increase the resistance of the battery so thatthe effective capacity of the battery rather decreases.

In both of the anode sheet made of the amorphous material and the anodesheet made of the carbon material in the methods described inJP-A-2000-182602 and JP-A-2001-15172, a bonding agent of the electrodeis in direct contact with the lithium metal foil so that the bondingagent reacts with part of the lithium metal foil to form a highlyresistant film.

Further, in the amorphous material sheet, the metal distributioninevitably becomes non-uniform in the microscopic scale resulting in thegeneration of the local concentration of the electric field. Because ofthese reasons, it is difficult to maintain the cycle performance at ahigher level.

The following descriptions can be found in the above publications withrespect to electrolytes. An electrolyte prepared by dissolving 0.4weight part of LiBF₄ and 12.1 weight part of LiPF₆ into a mixed solventcomposed of 65.5 weight part of diethyl carbonate and 22 weight part ofethylene carbonate is described in JP-A-2000-21392; an electrolyteprepared by dissolving 1 mol/liter of LiPF₆ into a mixed solventcomposed of ethylene carbonate and dimethyl carbonate in a volume ratioof 1:1 is described in JP-A-11(1999)-135120; an electrolyte prepared bydissolving 1 mol/liter of LiPF₆ into a mixed solvent composed ofethylene carbonate and diethyl carbonate in a volume ratio of 2:8 isdescribed in JP-A-10(1998)-21964; an electrolyte prepared by dissolving0.4 g of LiBF₄ and 12.1 g of LiPF₆ into a mixed solvent composed of 65.3g of diethyl carbonate and 22.2 g of ethylene carbonate is described inJP-A-2000-182602; and an electrolyte prepared by dissolving 0.4 g ofLiBF₄ and 12.1 g of LiPF₆ into a mixed solvent composed of 65.3 g ofdiethyl carbonate and 22.2 g of ethylene carbonate followed by furtherdissolution of an adding agent such as1,2-bis(ethoxycarbonyl)-1,2-dimethylhydrazine is described inJP-A-2001-15172. These electrolytes are described in the respectiveexamples of the publications. Further, various solvents are cited anddescribed to be used as an electrolyte in the bodies of thespecifications.

However, the detailed review with respect to the optimum value and rangeregarding the solvent composition of the electrolyte, the volume ratiofor mixing and the lithium salt concentration is not provided.

DISCLOSURE OF INVENTION

In view of the above problems possessed by the prior art, an object ofthe present invention is to provide a lithium secondary battery having asubstantially elevated battery capacity in a voltage range in which thebattery is actually used while maintaining a higher charge-dischargeefficiency and an excellent cycle performance.

The present invention provides a lithium secondary battery including (i)a cathode including a lithium-containing composite oxide, (ii) an anodewith a multi-layer structure including a first layer (21) having carbonas a main component, and a second layer (22) with lithium-ionconductivity having a material as a main component which can insert andextract lithium ion, and (iii) a non-aqueous electrolyte composed of amixed solvent including a first non-aqueous solvent having a specificdielectric constant of 30 or more and a viscosity of 1 cP or more and asecond non-aqueous solvent having a specific dielectric constant of 10or less and a viscosity below 1 cP in a volume ratio from 2:8 to 6:4,and dissolving therein a lithium salt in a range from 0.5 to 1.5mol/liter; and a lithium secondary battery including (i) a cathodeincluding a lithium-containing composite oxide, (ii) an anode with amulti-layer structure including a first layer having carbon as a maincomponent, a second layer with lithium-ion conductivity having amaterial as a main component which can insert and exract lithium ion,and a third layer which contains lithium and is not in direct contactwith the first layer, and (iii) a non-aqueous electrolyte composed of amixed solvent including a first non-aqueous solvent having a specificdielectric constant of 30 or more and a viscosity of 1 cP or more and asecond non-aqueous solvent having a specific dielectric constant of −10or less and a viscosity below 1 cP in a volume ratio from 2:8 to 6:4,and dissolving therein a lithium salt in a range from 0.5 to 1.5mol/liter.

The present invention will be described in detail.

As described in the prior art, it is conjectured that the compatibilityof the higher amount of the lithium insertion and extraction and thehigher charge-discharge efficiency can be theoretically possible byusing the anode prepared by combining the carbon material for anode andthe material having the larger amount of the lithium insertion andextraction such as the metal oxide. It is further considered that theadvance addition of the metal lithium by an amount equal to theirreversible capacity of the anode to the carbon material for anodereduces the irreversible capacity of the battery to increase its energydensity. However, in reality, the mere combination of these materialscan hardly increase the energy density of the battery as mentioned inthe section of the prior art.

Accordingly, in the present invention, the multi-layer structure is usedas the anode structure including the first layer having the carbon asthe main component, and (a) the second layer with the lithium-ionconductivity having the material as the main component which can insertand exract the lithium ion; or (b) the above second layer, and the thirdlayer which contains the lithium or a lithium-containing compound and isnot in direct contact with the first layer. That is, the layer havingthe carbon as the main component and the other layer having a materialother than carbon as the main component and inserting and extracting thelithium ion are formed in the respective multi-layer structures.

In the secondary battery of the present invention having the aboveconfiguration, the uniform presence of the active substance on the anodeachieved by using the filmy material uniformizes the electric fielddistribution between the cathode and the anode. Therefore, the localconcentration of the electric field hardly takes place, and the stablebattery performance can be obtained without occurrence of fractures suchas peeling-off of the active substance from a current collector. Whenthe electric field distribution is non-uniform, the lithium-insertinglayer may be locally swollen to cause the deterioration of the batteryperformance. Impurities such as a bonding agent may react with the metallithium to form the highly resistive film to worsen the batteryperformance. The anode of the present invention using the filmy materialcan solves these problems.

In case of the embodiment (b) including the third layer, the secondlayer between the first layer and the third layer suppresses the directreaction between the active sites on the carbon anode surface and themetal lithium so as to exert the added lithium to effectively compensatethe irreversible capacity of the carbon anode. Further, a part of theadded lithium is doped into the material with the lithium ionconductivity thereby increasing the lithium ion concentration. Theincrease of the number of the electronic charged carriers in the filmymaterial further elevates the lithium ion conductivity so that thebattery resistance can be reduced to further increase the effectivecapacity of the battery.

An anode for a lithium secondary battery can be also provided which isequipped with a region containing lithium an amount of which is largerthan the theoretical composition under the fully charged state.

When the anode having the second layer disposed between the first layerand the third layer is charged, a part of the lithium forming the thirdlayer is doped into the second layer. The phenomenon is utilized to dopethe lithium which exceeds the saturated amount into the second layerunder the fully charged state without consuming the lithium contained inthe cathode to provide the anode having the second layer doped with thelithium.

When the charging and the discharging are repeated on the multi-layeranode including the first to third layers, the lithium contained in thethird layer is doped into the first layer and the second layer, and thethird layer gradually disappears. During this step, the second layercontaining the lithium is generated. The anode having the second layerwhich contains the lithium itself contributes to the realization of thebattery having the excellent performance in the viewpoint different fromthat of the three-layer structure.

The lithium secondary battery generated in this manner enables theincrease of the effective capacity of the battery because the filmhaving the larger electric resistance is not formed, different from theprior art in which the metal lithium is in electric contact with theanode during the battery fabrication.

The structure formed by stacking the carbon-containing layer and thelithium-containing layer can realize a higher lithium-inserting amountby taking the advantages of the both layers without forming dendrite.

However, as described in the section of the prior art, the problem inconnection with the decrease of the battery performance arises in thestructure in which the carbon layer is in direct contact with thelithium layer because the lithium reacts with the carbon on theinterface to form the highly insulative film. Although the problem canbe alleviated by replacing the lithium layer with a lithium-alloy layer,the lithium in the lithium-alloy layer also reacts with the carbon layerto decrease the battery performance after all.

The second layer of the present invention in connection with the lithiumsecondary battery including the third layer originally has the lithiumion conductivity, and the doping of the lithium by means of the chargingand the discharging further elevates the lithium ion conductivity. Theabove film having the higher lithium ion conductivity never hinders thecharge-discharge reaction, and rather acts as a protective film tosuppress the side reaction between the electrolyte and the activesubstance, thereby improving the battery performance. The filmy secondlayer superposed on the first layer containing the carbon as the maincomponent, and the filmy third layer superposed on the second layersuppress the intercalation between the carbon layers under the situationwhere the lithium ion is solvated, thereby preventing the deteriorationof the carbon layers to attain the improvement of the cycle performance.

However, on the other hand, when only a non-aqueous solvent having aspecific dielectric constant of 30 or more and a viscosity of 1 cP ormore is used as an electrolyte solvent, the dispositions of the filmysecond layer on the first layer containing the carbon as the maincomponent and of the filmy third layer on the second layer hardlyimpregnate the electrolyte having the higher viscosity to the firstlayer, thereby increasing the resistance at the electrode interface sothat the sufficient battery performance cannot be extracted. Further,when only a non-aqueous solvent having a specific dielectric constant of10 or less and a viscosity less than 1 cP is used as an electrolytesolvent, the dissolution of the lithium salt is insufficient due to thelower specific dielectric constant though the solvent easily reaches tothe first layer. Accordingly, the lack of the ionic conductivity of theelectrolyte increases the internal resistance of the battery so that thesufficient battery performance cannot be extracted.

The investigations of the present inventors have revealed that theimpregnation of the electrolyte and the dissolution of the lithium saltcan be consistent with each other in the above-mentioned stackedstructure by using a non-aqueous electrolyte composed of a mixed solventincluding a first non-aqueous solvent having a specific dielectricconstant of 30 or more and a viscosity of 1 cP or more and a secondnon-aqueous solvent having a specific dielectric constant of 10 or lessand a viscosity below 1 cP in a volume ratio from 2:8 to 6:4, anddissolving therein a lithium salt in a range from 0.5 to 1.5 mol/liter.Accordingly, the non-aqueous electrolyte having the above composition isused in the present invention.

As described above, the consistence of the higher charge-dischargeefficiency and capacity can be possible and the excellent cycleperformance can be also realized in the present invention by applying,as the configuration of the anode, the multi-layer structure includingthe first layer having the carbon as the main component, and the secondlayer with the lithium-ion conductivity having the material as the maincomponent which can insert and extract the lithium ion; or themulti-layer structure including the above second layer, and the thirdlayer which contains the lithium or the lithium-containing compound andis not in direct contact with the first layer, and by optimizing thesolvent composition of the electrolyte, the mixing volume ratio and thelithium salt concentration for the anode configuration.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view exemplifying a lithium secondarybattery anode in accordance with a first embodiment of the presentinvention.

FIG. 2 is a longitudinal sectional view exemplifying a lithium secondarybattery anode in accordance with a second embodiment of the presentinvention.

FIG. 3 is a longitudinal sectional view exemplifying a lithium secondarybattery anode in accordance with a third embodiment of the presentinvention.

FIG. 4 is a longitudinal sectional view exemplifying a lithium secondarybattery anode in accordance with a fourth embodiment of the presentinvention.

FIG. 5 is a longitudinal sectional view exemplifying a lithium secondarybattery anode in accordance with a fifth embodiment of the presentinvention.

FIG. 6 is a sectional view exemplifying the structure of the secondarybattery of the present invention.

FIG. 7 is a view showing one example of a sectional structure of ananode of Comparative Example.

BEST MODE FOR IMPLEMENTING INVENTION

The shape of the secondary battery of the present invention is notespecially restricted and includes, for example, a cylindrical shape, asquare shape and a coin shape.

The first, second and third layers of the present invention contain thecarbon, the filmy lithium inserting material and the lithium or thelithium-containing material as the main components, respectively, andmay further contain an addition agent and so forth. The main componentof the present invention refers to a content from more than 50% inweight to 100%. The respective layers may be singular or plural, and thebelow configurations are not included in principle.

-   -   (i) A configuration in which the first layer and the third layer        are in direct contact with each other.    -   (ii) A configuration in which the first layer is disposed on the        outermost surface on the anode.

The order of the stacking is arbitrary except for the aboveconfigurations. For example, the second and third layers can be disposedon the respective top and bottom surfaces of the first layer to furtherelevate the battery capacity while maintaining the highercharge-discharge efficiency and the excellent cycle performance.

The second layer of the present invention has the filmy material havingthe lithium ion conductivity as the main component. The lithium ionconductivity refers to a property in which current flows in a substancewith lithium ion as a carrier of an electric charge.

The filmy material refers to, different from a particulate material, amaterial constituting the film with a nearly uniform composition, and isformed by using, for example, an evaporation method, a CVD method or asputtering method. For instance, a filmy material prepared bycoagulating a particulate material having the lithium ion conductivitywith a bonding agent is not included in the present invention.

The second layer is desirably made of the material inserting andextracting the lithium ion in addition to having the above-describedlithium conductivity. The material inserting and extracting the lithiumion refers to that taking the lithium into the material, and thetaking-in of the lithium includes the formation of alloy and a featuretaking the lithium in the structure without forming the alloy with thematerial.

Further, the second layer is preferably an amorphous structure. Sincethe doping and the dedoping of the lithium electrochemically to and fromthe amorphous structure take place at a potential lower than that of acrystalline structure, the battery capacity can be increased whilemaintaining the higher operating voltage and the higher charge-dischargeefficiency. The amorphous of the present invention refers to that havinga broad scattering band with a peak from 15 to 40 degree expressed as a“2θ” value in the X-ray diffraction method using CuK α rays.

The amorphous structure is crystallographically more isotropic than thecrystalline structure so that the strength against an external stressand the chemical stability are excellent. The second layer is hardlysubject to the influence of the expansion and the contraction of thecharging and the discharging of the anode, and the stability after therepetition of the charging and the discharging is excellent and thecapacity deterioration rarely takes place because the reaction with theelectrolyte rarely occurs.

The second layer is preferably formed by the evaporation method, the CVDmethod or the sputtering method. These film-forming methods provide theamorphous ion-conductive film uniformly on the anode. The filmuniformizes the electric field distribution between the cathode and theanode. Accordingly, no local concentration of the electric field takesplace, and the stable battery performance can be obtained withoutoccurrence of fractures such as peeling-off of the active substance fromthe current collector.

The material forming the second layer of the present invention is notespecially restricted provided that it has the lithium ion conductivityand inserts and extracts the lithium ion, and is preferably contains oneor more elements or their oxides selected from the group consisting ofSi, Ge, In, Sn and Pb. The selection of the above material having theamorphous structure can increase the battery capacity while maintainingthe higher operating voltage and the higher charge-discharge efficiency,and the fabrication becomes easier. Especially, Si, Sn and their oxidescan provide the excellent cycle performance because of the smallstructural change after the insertion of the lithium and littledeterioration after the repetition of the charging and the discharging.

The material forming the third layer of the present invention is notespecially restricted provided that it is the lithium or thelithium-containing compound, and is preferably the metal lithium, thelithium alloy, lithium nitride, Li_(3·x)M_(x)N (M=Co, Ni, Cu) or theirmixture. Such the material can compensate the irreversible capacity ofthe anode to elevate the charge-discharge efficiency because thematerial can electrochemically extract much lithium.

The third layer is preferably made of a material having an amorphousstructure. The amorphous structure is crystal-lographically moreisotropic than the crystalline structure so that the strength against anexternal stress and the chemical stability are excellent, and theamorphous structure hardly reacts with the electrolyte to take place aside reaction. Accordingly, the lithium contained in the third layer isefficiently utilized to compensate the irreversible capacity of theanode. The third layer is preferably formed by the evaporation method,the CVD method or the sputtering method. These film-forming methods canprovide the uniform amorphous layer on the entire anode. A side reactionhardly takes place because the use of a solvent is not required, and apurer layer can be formed. The lithium contained in the third layer isefficiently utilized to compensate the irreversible capacity of theanode.

FIG. 1 is a longitudinal sectional view exemplifying a lithium secondarybattery anode having a multi-layer structure in accordance with a firstembodiment of the present invention, and FIG. 2 is a longitudinalsectional view exemplifying a lithium secondary battery anode having amulti-layer structure in accordance with a second embodiment of thepresent invention.

FIG. 1 exemplifies the anode having the multi-layer structure includinga first layer (21) having carbon as a main component, and a second layer(22) with lithium-ion conductivity having a material as a main componentwhich can insert and extract lithium ion, sequentially stacked on ananode current collector (20). FIG. 2 exemplifies the anode having themulti-layer structure including the first layer (21) having the carbonas the main component, the second layer (22) with the lithium-ionconductivity having the material as the main component which can insertand extract the lithium ion, and further a third layer (23) made oflithium or a lithium-containing material sequentially stacked on theanode current collector (20), and the first layer (21) and the thirdlayer (23) are not in direct contact with each other.

The current collector (20) is an electrode which takes the current outof the battery and takes the current into the battery during thedischarging and the charging, respectively. The current collector (20)may be an electro-conductive foil made of metal such as aluminum,copper, stainless steel, gold, tungsten and molybdenum.

The first layer (21) is the carbon anode layer inserting and extractingthe lithium during the charging and the discharging, respectively. Thecarbon anode (21) is made of the carbon which can insert the lithium,and graphite, amorphous carbon, fullerene, carbon nanotube, DLC or theirmixture can be used.

The anode second layer (22) having the lithium ion conductivity is madeof the material which can insert and extract the lithium ion. Such thematerial includes boron oxide, phosphorous oxide, aluminum oxide andtheir composite oxide in addition to the above-described Si, Ge, In, Sn,Pb and their oxides, and one or more of these materials can be usedsingly or in combination. Lithium halide or lithium chalcogenide may beadded thereto to increase the lithium ion conductivity.

The materials are preferably amorphous as described above. The use ofthe amorphous material can reduce the potentials for the doping and thededoping of the lithium compared with the crystal, thereby resulting inthe increase of the operating voltage of the battery. As mentionedbefore, the anode second layer (22) is preferably formed by using theCVD method, the evaporation method or the sputtering method. The use ofthese methods can form the amorphous layer with a uniform film propertyand uniform thickness. The anode second layer (22) can be doped with B,P, As or Sb to decrease its resistivity.

The anode third layer (23) is made of the lithium or thelithium-containing compound. Such a material includes metal lithium,lithium alloy, lithium nitride, Li_(3·x)M_(x)N (M=Co, Ni, Cu) and theirmixture, and one or more of these materials can be used singly or incombination. The material is preferably amorphous. The use of theamorphous material suppresses the side reaction with the electrolyte toutilize the lithium contained 21, in the material for the effectivecompensation of the irreversible capacity. As a result, the initialcharge-discharge efficiency of the battery is elevated to increase theenergy density. The anode third layer (23) is preferably formed by usingthe CVD method, the evaporation method or the sputtering method. The useof these methods can form the amorphous layer with a uniform filmproperty and uniform thickness.

FIG. 3 is a longitudinal sectional view exemplifying a lithium secondarybattery anode in accordance with a third embodiment of the presentinvention, and FIG. 4 is a longitudinal sectional view exemplifying alithium secondary battery anode having a multi-layer structure inaccordance with a fourth embodiment of the present invention.

The embodiment of FIG. 3 similar to the first embodiment shown in FIG. 1is a structure in which anode first layers (21) made of carbon and anodesecond layers (22) are sequentially stacked on both surfaces of acurrent collector (20). The embodiment of FIG. 4 similar to the secondembodiment shown in FIG. 2 is a structure in which anode first layers(21) made of carbon, anode second layers (22) and anode third layers(23) are sequentially stacked on both surfaces of a current collector(20).

FIG. 5 is a longitudinal sectional view exemplifying a lithium secondarybattery anode having a multi-layer structure in accordance with a fifthembodiment of the present invention. In the embodiment, an anode firstlayer (21) is formed on a current collector (20), and a saturatedlithium layer (24) is formed thereon. In the saturated lithium layer(24), a region containing lithium which exceeds the saturated amount ina fully charged state or a region containing the lithium in excess ofthe theoretical composition is formed. The saturated amount (thetheoretical composition) of the lithium refers to the maximum value ofthe lithium containable in a compound which is generated by a substanceand the lithium. The saturated lithium layer (24) corresponds to theanode second layers (22) of the first to fourth embodiments and isincluded in the present invention.

The lithium saturated amounts of various lithium alloys are describedin, for example, “Denshi Zairyo” (April issue, 2001, vol.20, No.4, page78, published on Apr. 1, 2001; published by Kogyo Chosakai PublishingCo., Ltd.). The below-described values are the upper limits of thelithium alloy compositions, and alloys containing the lithium exceedingthese composition ratios are not obtainable by using an ordinary methodof fabricating alloy.

-   -   LiSi alloy: Li₄Si    -   LiAl alloy: LiAl    -   LiSn alloy: Li_(4.4)Sn    -   LiCd alloy: Li₃Cd    -   LiSb alloy: Li₃Sb    -   LiPb alloy: Li_(4.4)Pb    -   LiZn alloy: LiZn    -   LiBi alloy: Li₃Bi

The anode of FIG. 5 includes the saturated lithium layer (24) containingthe lithium compound having the saturated amount of the lithium. Thelithium compound can be obtained by conducting the charging and thedischarging of the anode having the structure shown in FIG. 2 underspecified conditions. Although the saturated lithium layer (24) isexemplified to be uniformly formed on the anode first layer (21) made ofthe carbon in FIG. 5, another structure in which a lithium-containinglayer is formed on the saturated lithium layer (24) is also included inthe present invention.

As described above, the electrolyte of the present invention is thenon-aqueous electrolyte composed of the mixed solvent composed of thefirst non-aqueous solvent and the second non-aqueous solvent in thevolume ratio from 2:8 to 6:4, and dissolving therein the lithium salt inthe range from 0.5 mol/liter to 1.5 mol/liter. When the ratio of thefirst non-aqueous solvent is below 2, the specific dielectric constantof the mixed solvent is lower to make the dissociation insufficientresulting in the lack of the ionic conductivity of the electrolyte. Thelack of the conductivity increases the internal resistance of thebattery not to sufficiently draw out the battery performance. When theratio of the first non-aqueous solvent exceeds 6, the viscosity of theelectrolyte becomes higher to hardly achieve the penetration of theelectrolyte to the first layer, thereby increasing the electrodeinterface resistance not to sufficiently draw out the batteryperformance. The lithium salt concentration out of the above regionlacks the electro-conductivity of the electrolyte, thereby increasingthe internal resistance of the battery not to sufficiently draw out thebattery performance. The especially preferable mixed volume ratiobetween the first non-aqueous solvent and the second non-aqueous solventis from 3:7 to 5:5, and the especially preferable concentration of thelithium salt is from 0.8 mol/liter to 1.2 mol/liter.

Ethylene carbonate, propylene carbonate and butylene carbonate areexemplified as the first non-aqueous solvent of the present invention,and 1,2-dimethoxyethane, dimethyl carbonate, methylethyl carbonate anddiethyl carbonate are exemplified as the second non-aqueous solvent.

As the lithium salt of the present invention, LiBF₄, LiPF₆, LiCl, LiBr,LiI, LiN(CF₃SO₂)₂ and LiN(C₂F₅SO₂)₂ are exemplified.

The cathode of the present invention can be used which is prepared byapplying, on a substrate such as an aluminum foil, a mixture formed bydispersing a lithium-containing composite oxide having a formula ofLi_(x)MO₂ (M is at least one transition metal) such as Li_(x)CoO₂,Li_(x)NiO₂, Li_(x)Mn₂O₄, Li_(x)MnO₃ and Li_(x)Ni_(y)C_(1·y)O₂, anelectro-conductive substance such as carbon black, and a bonding agentsuch as poly-fluorinated vinylidene in a solvent such asN-methyl-2-pyrrolidone.

As a separator of the present invention, polyolefin such aspolypropylene and polyethylene, and a porous film such as fluorocarbonresin can be used.

FIG. 6 is a sectional view exemplifying the structure of the secondarybattery of the present invention. In the drawing, the cathode isfabricated by forming a cathode active substance-containing layer (12)on a cathode current collector (11), and an anode is fabricated byforming an anode active substance-containing layer (13) on an anodecurrent collector (14). The cathode and the anode are opposed to eachother sandwiching an electrolyte (30) and a porous separator (50) in theelectrolyte (30). The porous separator (50) is disposed nearly parallelto the anode active substance-containing layer (13).

EXAMPLES

Although the present invention will be hereinafter described more indetail by showing Examples, the present invention is not restricted tothese Examples unless departing from the gist thereof.

[Fabrication Example of Cathode Sheet]

A cathode sheet was fabricated by applying slurry prepared by dispersingand mixing Li_(1.1)Mn₂O₄, a conductivity supplying agent andpoly-fluorinated vinylidene in N-methyl-2-pyrrolidone, on an aluminumfoil having thickness of 20 μm and acting as a cathode current collector(11) followed by drying. The cathode sheet after the drying wascompressed by using a pressing machine.

[Fabrication Example of Anode First Layer]

An anode first layer was fabricated by applying slurry prepared bydispersing and mixing graphite powders, a conductivity supplying agentand poly-fluorinated vinylidene in N-methyl-2-pyrrolidone, on a copperfoil having thickness of 10 μm and acting an anode current collector(20) followed by drying.

The anode first layer after the drying was compressed by using apressing machine.

[First Fabrication Example of Anode Second Layer]

A Si layer acting as an anode second layer was formed on the anode firstlayer fabricated in the Fabrication Example of Anode First Layer byusing a vacuum-deposition method to provide an anode sheet having astacked structure including the anode current collector, the anode firstlayer and the anode second layer in this turn.

[Second Fabrication Example of Anode Second Layer]

A Si layer acting as an anode second layer was formed on the anode firstlayer fabricated in the Fabrication Example of Anode First Layer byusing a sputtering method to provide an anode sheet having a stackedstructure including the anode current collector, the anode first layerand the anode second layer in this turn.

[Third Fabrication Example of Anode Second Layer]

A boron oxide layer acting as an anode second layer was formed on theanode first layer fabricated in the Fabrication Example of Anode FirstLayer by using a vacuum-deposition method to provide an anode sheethaving a stacked structure including the anode current collector, theanode first layer and the anode second layer in this turn.

[First Fabrication Example of Anode Third Layer]

A metal lithium layer acting as an anode third layer was formed on theanode second layer of the anode sheet fabricated in the FirstFabrication Example of Anode Second Layer by using a vacuum-depositionmethod to provide an anode sheet having a stacked structure includingthe anode current collector, the anode first layer, the anode secondlayer and the anode third layer in this turn.

[Second Fabrication Example of Anode Third Layer]

A metal lithium layer acting as an anode third layer was formed on theanode second layer of the anode sheet fabricated in the SecondFabrication Example of Anode Second Layer by using a vacuum-depositionmethod to provide an anode sheet having a stacked structure includingthe anode current collector, the anode first layer, the anode secondlayer and the anode third layer in this turn.

[Third Fabrication Example of Anode Third Layer]

A metal lithium layer acting as an anode third layer was formed on theanode second layer of the anode sheet fabricated in the FabricationExample of Anode Second Layer-3 by using a 28, vacuum-deposition methodto provide an anode sheet having a stacked structure including the anodecurrent collector, the anode first layer, the anode second layer and theanode third layer in this turn.

[First Comparable Example of Fabricating Anode Sheet]

An anode sheet for Comparable Example shown in FIG. 7 and formed bycovering an anode current collector (20) with a first layer (21)containing dispersed Si powders (26) was fabricated by applying slurryprepared by dispersing and mixing graphite powders, Si powders, aconductivity supplying agent and poly-fluorinated vinylidene inN-methyl-2-pyrrolidone, on a copper foil having thickness of 10 μm andacting as an anode current collector (20) followed by drying. The anodesheet for Comparable Example after the drying was compressed by using apressing machine.

[Second Comparable Example of Fabricating Anode Sheet]

A metal lithium layer was formed on the anode first layer fabricated inthe Fabrication Example of Anode First Layer by using avacuum-deposition method to provide an anode sheet for ComparativeExample having a stacked structure including the anode currentcollector, the anode first layer and the metal lithium layer in thisturn.

Example 1

A battery having a structure exemplified in FIG. 6 was fabricated bystacking the anode sheet fabricated in the First Fabrication Example ofAnode Second Layer, a separator made of polypropylene non-woven fabricand the cathode sheet fabricated in the Fabrication Example of CathodeSheet. An electrolyte used was prepared by dissolving 1.0 mol/liter ofLiPF₆ in a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 3:7).

Example 2

A battery was fabricated under the same conditions as those of Example 1except that the anode sheet fabricated in Second Fabrication Example ofAnode Second Layer was used in the anode.

Example 3

A battery was fabricated under the same conditions as those of Example 1except that the anode sheet fabricated in Third Fabrication Example ofAnode Second Layer was used in the anode.

Example 4

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 2:8) into which 1.0 mol/liter of LiPF₆was dissolved was used.

Example 5

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 6:4) into which 1.0 mol/liter of LiPF₆was dissolved was used.

Example 6

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 3:7) into which 0.5 mol/liter of LiPF₆was dissolved was used.

Example 7

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 3:7) into which 1.5 mol/liter of LiPF₆was dissolved was used.

Example 8

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and dimethylcarbonate (mixed volume ratio is 3:7) into which 1.0 mol/liter of LiPF₆was dissolved was used.

Example 9

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate, diethylcarbonate and methylethyl carbonate (mixed volume ratio is 3:5:2) intowhich 1.0 mol/liter of LiPF₆ was dissolved was used.

Example 10

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate, diethylcarbonate and methylethyl carbonate (mixed volume ratio is 3:2:5) intowhich 1.0 mol/liter of LiPF₆ was dissolved was used.

Example 11

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate, propylenecarbonate and diethyl carbonate (mixed volume ratio is 30:5:65) intowhich 1.0 mol/liter of LiPF₆ was dissolved was used.

Example 12

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 3:7) into which 1.0 mol/liter ofLiN(C₂F₅SO₂)₂ was dissolved was used.

Example 13

A battery was fabricated under the same conditions as those of Example 1except that the anode sheet fabricated in First Fabrication Example ofAnode Third Layer was used in the anode.

Example 14

A battery was fabricated under the same conditions as those of Example13 except that the anode sheet fabricated in Second Fabrication Exampleof Anode Third Layer was used in the anode.

Example 15

A battery was fabricated under the same conditions as those of Example13 except that the anode sheet fabricated in Third Fabrication Exampleof Anode Third Layer was used in the anode.

Example 16

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 2:8) into which 1.0 mol/literof LiPF₆ was dissolved was used.

Example 17

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 6:4) into which 1.0 mol/literof LiPF₆ was dissolved was used.

Example 18

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 3:7) into which 0.5 mol/literof LiPF₆ was dissolved was used.

Example 19

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 3:7) into which 1.5 mol/literof LiPF₆ was dissolved was used.

Example 20

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddimethyl carbonate (mixed volume ratio is 3:7) into which 1.0 mol/literof LiPF₆ was dissolved was used.

Example 21

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate, diethylcarbonate and methylethyl carbonate (mixed volume ratio is 3:5:2) intowhich 1.0 mol/liter of LiPF₆ was dissolved was used.

Example 22

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate, diethylcarbonate and methylethyl carbonate (mixed volume ratio is 3:2:5) intowhich 1.0 mol/liter of LiPF₆ was dissolved was used.

Example 23

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate, propylenecarbonate and diethyl carbonate (mixed volume ratio is 30:5:65) intowhich 1.0 mol/liter of LiPF₆ was dissolved was used.

Example 24

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 3:7) into which 1.0 mol/literof LiN(C₂F₅SO₂)₂ was dissolved was used.

Comparative Example 1

A battery was fabricated under the same conditions as those of Example 1except that the anode sheet fabricated in Fabrication Example of AnodeFirst Layer was used in the anode.

Comparative Example 2

A battery was fabricated under the same conditions as those of Example 1except that the anode sheet fabricated in First Comparable Example ofFabricating Anode Sheet was used in the anode.

Comparative Example 3

A battery was fabricated under the same conditions as those of Example 1except that the anode sheet fabricated in Second Comparable Example ofFabricating Anode Sheet was used in the anode.

Comparative Example 4

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and propylenecarbonate (mixed volume ratio is 1:1) into which 1.0 mol/liter of LiPF₆was dissolved was used.

Comparative Example 5

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 1:9) into which 1.0 mol/liter of LiPF₆was dissolved was used.

Comparative Example 6

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 7:3) into which 1.0 mol/liter of LiPF₆was dissolved was used.

Comparative Example 7

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 3:7) into which 0.4 mol/liter of LiPF₆was dissolved was used.

Comparative Example 8

A battery was fabricated under the same conditions as those of Example 1except that a mixed solvent composed of ethylene carbonate and diethylcarbonate (mixed volume ratio is 3:7) into which 1.6 mol/liter of LiPF₆was dissolved was used.

Comparative Example 9

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate andpropylene carbonate (mixed volume ratio is 1:1) into which 1.0 mol/literof LiPF₆ was dissolved was used.

Comparative Example 10

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 1:9) into which 1.0 mol/literof LiPF₆ was dissolved was used.

Comparative Example 11

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 7:3) into which 1.0 mol/literof LiPF₆ was dissolved was used.

Comparative Example 12

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 3:7) into which 0.4 mol/literof LiPF₆ was dissolved was used.

Comparative Example 13

A battery was fabricated under the same conditions as those of Example13 except that a mixed solvent composed of ethylene carbonate anddiethyl carbonate (mixed volume ratio is 3:7) into which 1.6 mol/literof LiPF₆ was dissolved was used.

Charge-discharge cycle tests were conducted on the batteries of Examples1 to 24 and Comparative Examples 1 to 13 at test voltages in a rage of3.0 to 4.3 V. The results of the initial charge-discharge and thecapacity retention rate after 300 cycles with respect to the initialdischarging capacity are shown in Table 1 for Examples and in Table 2for Comparative Examples. TABLE 1 Capacity Initial Initial Charge-Retention Charging Discharging Discharge Rate After Capacity CapacityEfficiency 300 Cycles (mAh) (mAh) (%) (%) Example 1 155 145 93.7 89.5Example 2 154 144 93.6 88.6 Example 3 151 140 92.9 90.1 Example 4 152141 92.6 87.3 Example 5 150 139 92.8 86.9 Example 6 149 138 92.5 85.6Example 7 153 142 93.1 87.4 Example 8 152 142 93.3 89.1 Example 9 154142 93.1 90.6 Example 10 153 142 92.9 89.8 Example 11 152 142 93.6 90.0Example 12 154 144 93.2 89.6 Example 13 156 154 98.9 90.3 Example 14 152149 98.3 90.0 Example 15 153 150 98.2 90.2 Example 16 151 148 98.1 88.6Example 17 153 150 98.3 87.7 Example 18 150 147 98.0 85.6 Example 19 152149 98.2 88.0 Example 20 154 152 98.7 89.8 Example 21 155 153 98.6 89.9Example 22 150 148 98.4 90.2 Example 23 156 154 98.6 90.6 Example 24 151148 98.3 88.7

TABLE 2 Capacity Initial Initial Charge- Retention Charging DischargingDischarge Rate After Capacity Capacity Efficiency 300 Cycles (mAh) (mAh)(%) (%) Comp. Ex. 1 152 142 93.2 83.1 Comp. Ex. 2 148 89 60.2 32.6 Comp.Ex. 3 146 106 72.6 18.5 Comp. Ex. 4 86 27 31.6 Upto 155 cycles Comp. Ex.5 75 22 29.6 Upto 135 cycles Comp. Ex. 6 135 61 45.2 Upto 178 cyclesComp. Ex. 7 122 59 48.1 Upto 122 cycles Comp. Ex. 8 138 113 81.6 42.3Comp. Ex. 9 77 22 28.2 Upto 118 cycles Comp. Ex. 10 69 18 25.5 Upto 98cycles  Comp. Ex. 11 127 54 42.5 Upto 138 cycles Comp. Ex. 12 116 5043.2 Upto 161 cycles Comp. Ex. 13 130 103 79.5 38.5

The charge-discharge efficiency of the battery of Comparative Example 1using only the carbon material as the anode was 93.2%. On the otherhand, the charge-discharge efficiencies of the batteries of Examples 1to 3 forming the anode second layers made of the Si or the born oxidewere also around 93%. Accordingly, it is apparent that the latter wasnot inferior to the former. The charge-discharge efficiencies of thebatteries of Examples 13 to 15 forming the anode third layers made ofthe metal lithium were as high as 98.2% to 98.9% demonstrating that thecompensation of the irreversible capacity of the anode by the anodethird layer efficiently proceeded. On the other hand, in the battery ofComparative Example 2 in which the Si powders were dispersed in thecarbon material and in the battery of Comparative Example 3 in which themetal lithium layer was formed directly on the surface of the carbonmaterial, the charge-discharge efficiencies thereof were 60.2% and72.6%, respectively, demonstrating that the elevation of thecharge-discharge efficiencies could not be implemented by using theconfiguration dispersing the crystalline material in the carbon materialand the configuration forming the lithium layer on the carbon material.Further, in connection with the capacity retention rate after 300cycles, while the capacities of about 88.6% to 90.3% with respect to theinitial discharging capacities were retained in Examples 1 to 3 and 13to 15, only 32.6% and 18.5% could be retained in Comparative Examples 2and 3, respectively, demonstrating that the elevation of thecharge-discharge efficiencies could not be implemented by using theconfigurations of Comparative Examples.

The contributing factors seem that the electric contact of the anodelayer was lost to increase the resistance by the swelling and theconstriction due to the charging and the discharging of the Si powdersin Comparative Example 2, and the lithium formed on the carbon materialreacted with active sites on the carbon surface to form a highlyresistant film in Comparative Example 3.

The results of Examples 1 to 3 and 13 to 15 indicate that the secondarybatteries having the anode configurations of the present invention hadthe higher capacities and charge-discharge efficiencies and the stablecycle performances.

In Examples 1 to 24 in which the compositions of the electrolytesolvents, the mixed volume ratios and the lithium salt concentrationswere adjusted in the ranges of the present invention, the initialcharge-discharge efficiencies were as high as about 92 to 99%, and thecapacity retention rates after 300 cycles were retained at about 85 to90%. On the other hand, in Comparative Examples 4 to 13 in which thecompositions of the electrolyte solvents, the mixed volume ratios andthe lithium salt concentrations were out of the ranges of the presentinvention, the charge-discharge efficiencies of about 80% were retainedat the lithium salt concentrations of 1.6 mol/liter (ComparativeExamples 8 and 13), but the efficiencies were as bad as about 25 to 48%in the other Comparative Examples. Further, in connection with the cycleperformance, the capacities could not be retained after 300 cyclesexcept for Comparative Examples 8 and 13, and even in ComparativeExamples 8 and 13, the capacity retention rates were as low as around40%. The contributing factor seems that the electrode interfaceresistance or the internal resistance of the battery was increased sothat the sufficient battery performance cannot be extracted in thecompositions of the electrolyte solvents, the mixed volume ratios andthe lithium salt concentrations out of the ranges of the presentinvention.

The results of Examples 1 to 24 have indicated that the ranges of thecomposition of the solvent, of the mixed volume ratio and of the lithiumsalt concentration of the present invention were effective for the anodeconfiguration of the present invention.

Since the above embodiments are described only for examples, the presentinvention is not limited to the above embodiments and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

1. (canceled)
 2. A lithium secondary battery comprising: (i) a cathode including a lithium-containing composite oxide, (ii) an anode having a multi-layer structure including a first layer (21) having carbon as a main component thereof, a second layer (22) having a lithium-ion conductivity and including a material as a main component thereof which can insert and extract lithium ions, and a third layer (23) which contains lithium and is not in direct contact with the first layer (21), and (iii) a non-aqueous electrolyte composed of a mixed solvent including a first non-aqueous solvent having a specific dielectric constant of 30 or more and a viscosity of 1 cP or more and a second non-aqueous solvent having a specific dielectric constant of 10 or less and a viscosity below 1 cP in a volume ratio from 2:8 to 6:4, and dissolving therein a lithium salt in a range from 0.5 to 1.5 mol/liter.
 3. The lithium secondary battery as defined in claim 2, wherein the second layer (22) includes one or more components selected from the group consisting of Si, Ge, Sn, In, Pd and their oxides.
 4. The lithium secondary battery as defined in any one of claims 2 or 3, wherein the second layer (22) includes an amorphous structure formed by an evaporation method, a CVD method or a sputtering method.
 5. The lithium secondary battery as defined in any one of claims 2 to 4, wherein the third layer (23) includes one or more materials selected from the group consisting of metal lithium, lithium alloy and lithium nitride.
 6. The lithium secondary battery as defined in any one of claims 2 to 5, wherein the third layer (23) includes an amorphous structure formed by an evaporation method, a CVD method or a sputtering method.
 7. The lithium secondary battery as defined in any one of claims 2 to 6, wherein the first non-aqueous solvent includes one or more non-aqueous solvents selected from the group consisting of ethylene carbonate, propylene carbonate and butylene carbonate, and the second non-aqueous solvent includes one or more non-aqueous solvents selected from the group consisting of 1,2-dimethoxyethane, dimethyl carbonate, methylethyl carbonate and diethyl carbonate.
 8. The lithium secondary battery as defined in any one of claims 2 to 7, wherein the lithium salt includes one or more lithium salts selected from the group consisting of LiBF4, LiPF6, LiCl, LiBr, LiI, LiN(CF3SO2)2 and LiN(C2F5SO2)2.
 9. A method for fabricating a lithium secondary battery comprising the steps of: charging and/or discharging an anode of a lithium secondary battery including (i) a cathode having a lithium-containing composite oxide, (ii) the anode having a multi-layer structure including a first layer (21) having carbon as a main component thereof, a second layer (22) having a lithium-ion conductivity and including a material as a main component thereof which can insert and extract lithium ions, and a third layer (23) which contains lithium and is not in direct contact with the first layer (21), and (iii) a non-aqueous electrolyte, thereby doping the lithium contained in the third layer to the second layer to form the second layer containing the lithium. 